Recently, networks and IT technique are rapidly spread all over the world, which requires security technique such as the encryption technique and the authentication technique. As one element of these techniques, random numbers are often used. Currently, random number generating circuits based on several types of basic principles have come into practical use. “Toshiba Review” Vol. 58.8 (2003) (first prior art) is an example of ultra-small-size true random number generating circuit for sophisticated information security. Moreover, JP-A-2003-173254 (second prior art) discloses a random number generating circuit using an ambiguous output obtained by turning power supply of an RS flip-flop ON/OFF.